This invention relates to the field of sensors, and more particularly, to solid-state sensors that use spaced electrodes to measure parameters such as pressure, acceleration, etc. This invention also relates to methods for fabricating such devices.
Interest in the development of solid-state sensors has increased dramatically in recent years. Solid-state sensors, and in particular solid-state pressure sensors, have found wide spread application in many areas including industrial process controls, automobile engine monitoring, and biomedicine. Solid-state pressure sensors typically include a sealed cavity with a diaphragm that deforms geometrically in response to an applied pressure or pressures. Many times, the construction of the sealed cavity involves micro-machining a substrate to form one or more silicon diaphragms and bonding the micro-machined substrate to a sensor substrate.
For many pressure sensors, the magnitude of the geometric deformation or flexure of the diaphragm is proportional to the applied pressure. There are a number of approaches for measuring the degree that the diaphragm deforms or flexes. One approach is to diffuse a piezoresistive-type resistor into the diaphragm. By measuring the stress-induced changes in the resistivity of the piezoresistive resistors, the flexure of the diaphragm can be determined. Another approach is to provide a conductive plate adjacent to the diaphragm. By applying an AC signal between the diaphragm and the conductive plate, the capacitance between the diaphragm and a conductive plate can be measured. The amount of capacitance is related to the degree that the diaphragm is deformed.
Another approach is to use a Movable Gate Field Effect Transistor (MOGFET), as suggested in U.S. Pat. No. 4,812,888 to Blackburn. Blackburn suggests a capacitive pressure transducer that uses a field-effect transistor (FET) to sense the flexure of the diaphragm. In Blackburn, the diaphragm is conductive and serves as the gate of the field effect transistor (FET). As the diaphragm deforms under an applied pressure, a portion of the diaphragm moves closer to the source and drain of the FET, which are fabricated in the substrate. This changes the resistivity of the channel that lies between the source and the drain, thereby changing the source-drain current of the FET. The amount that the diaphragm deforms can be determined by measuring the source-drain current.
Other references that disclose sensors that use a Movable Gate Field Effect Transistor (MOGFET) include U.S. Pat. No. 4,873,871 to Bai et al.; U.S. Pat. No. 5,668,033 to Ohara et al.; U.S. Pat. No. 5,578,843 to Garabedian et al.; U.S. Pat. No. 5,572,057 to Yamamoto et al.; U.S. Pat. No. 5,541,437 to Watanabe et al.; U.S. Pat. No. 5,500,549 to Takeuchi et al.; U.S. Pat. No. 5,504,356 to Takeuchi et al.; and U.S. Pat. No. 5,503,017 to Mizukoshi et al.
A limitation of many of these prior art sensor devices is that the electric field between the diaphragm and the substrate is not uniform across the diaphragm, particularly as the diaphragm deforms toward or away from the substrate. In many cases, the diaphragm is only supported around its periphery, and the center tends to move toward or away from the substrate more than the rest of the diaphragm. This tends to produce a higher electric field at the center of the diaphragm than around the edges, and this differential increases with increased diaphragm displacement. Typically, this causes the output of a capacitive, MOGFET, or other sensor type that is sensitive to the electric field between the diaphragm and the substrate to have a more complex behavior than if a uniform electric field extended across the diaphragm. In addition, the efficiency of the sensor may be reduced relative to a sensor that has a uniform electric field between the diaphragm and the substrate. What would be desirable, therefore, is a sensor device that provides a uniform electric field between the diaphragm and the substrate, regardless of the diaphragm displacement. This may have a number of advantages including simplifying the behavior and increasing the efficiency of the sensor.
Another limitation is that many of the prior art sensor devices inherently provide a pressure transducer, rather than a pressure switch. For many pressure sensing applications, detailed pressure values are not required as provided by, for example, a pressure transducer. Instead, only a few specific pressure thresholds often need to be detected, such as when the oil pressure of an engine drops below a predetermined threshold. To accommodate some of these applications, many of the prior art sensor devices use a typical pressure transducer in conjunction with an electronic threshold detector circuit. It would be desirable, therefore, to provide a pressure sensor that electro-mechanically provides a switch function so that expensive signal processing circuits are not required.
The present invention overcomes many of the disadvantages of the prior art by providing a sensor device that provides a relatively uniform electric field across a diaphragm, regardless of the displacement of the diaphragm. Preferably, this is accomplished by providing a uniform lateral spacing between the diaphragm and the substrate, over a selected range of diaphragm displacements. In one illustrative embodiment, a double layer diaphragm is provided including an upper support member and a lower electrode plate. The lower electrode plate is preferably attached to the upper support member by a post member, and the post member is preferably only attached to the center of the support member.
Using this configuration, any number of sensor types can be constructed including pressure sensors, acceleration sensors, etc. In an illustrative embodiment, a pressure transducer is provided wherein the support member forms one side of a sensor cavity. When a pressure is applied, the support member deforms. Since the electrode plate is only connected to the center region of the support member, the electrode plate does not substantially deform when the support member deforms under the applied pressure. Instead, the electrode plate tends to remain substantially non-deformed and substantially parallel to the substrate. Accordingly, the electric field between the electrode plate and the substrate remains relatively uniform across the electrode plate, regardless of the displacement of the diaphragm. This may simplify the behavior and increase the efficiency of the pressure transducer device.
The uniform electric field produced by the above illustrative embodiments can be sensed in any number of ways including, for example, using a capacitive type sensor, a MOGFET type sensor, etc. To form a capacitive type sensor, the sensor substrate may have an implant region under the electrode plate, wherein the implant region makes the substrate conductive. Alternatively, or in addition to, a metal layer may be deposited on the substrate under the electrode plate. In either case, the electrode plate is separated from the substrate and/or metal layer by a dielectric layer such as an oxide layer, an air layer, and/or a vacuum layer. By applying an AC signal between the electrode plate and the substrate and/or metal layer, the capacitance therebetween can be measured. The measured capacitance can be used to determine the spacing, and thus the desired sensor parameter.
To form a MOGFET type sensor, the substrate may have a source implant region and a drain implant region. A gap is positioned between the source implant region and the drain implant region. In this configuration, the electrode plate forms the gate of the MOGFET device. By applying a gate voltage to the electrode plate (e.g., gate) and a source-drain voltage to the source and drain implants, the spacing between the electrode plate and the substrate can be determined by measuring the resulting source-drain current. The gap may or may not include an implant.
In another illustrative embodiment, a pressure switch may be provided. The pressure switch detects when a threshold pressure of a gas or a fluid occurs and turns on one or more electric switches to activate a control device, alarm, or the like. Unlike a pressure transducer, the output of the pressure switch is ideally either xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d. In an illustrative pressure switch embodiment, the electrode plate and the substrate are conductive and have a voltage applied therebetween. The voltage causes an electrostatic force between the substrate and the electrode plate, wherein the electrostatic force increases as the electrode plate moves closer to the substrate. The support member also provides a deformation force that is opposite to the applied pressure and to the electrostatic force. The deformation force also increases as the support plate moves closer to the substrate. The electrode plate may snap toward the substrate when the externally generated force and the electrostatic force overcome the deformation force. This snapping action can be used effectively to form a switch. The electrode plate may maintain a uniform lateral spacing between the itself and the substrate, or may be a more conventional single-piece diaphragm.
In one illustrative embodiment, a dielectric layer is provided between the electrode plate and the substrate to prevent a direct electrical connection when the diaphragm snaps toward the substrate. This dielectric layer may be a dielectric substance such as an oxide, or may be a gas, a vacuum, or any other layer that provides electrical insulation.
For a capacitive type sensor, the snapping action of the electrode plate, as described above, may provide a transition from a lower capacitance state to a higher capacitance state. For a MOGFET type sensor, the snapping action of the electrode plate may provide a transition from an xe2x80x9coffxe2x80x9d FET device to an xe2x80x9conxe2x80x9d FET device.
It is recognized that the support member and the electrode plate may be designed so that little or no snapping action occurs. This may be accomplished by, for example, increasing the thickness, reducing the diameter of the support member, or decreasing the voltage applied to the electrode plate. By providing little or no snapping action, a transducer type sensor may be produced as described above.
An illustrative method for forming the two layer diaphragm structure discussed above includes: providing a substrate having an upper surface; forming a first sacrificial layer on the upper surface of the substrate; forming an electrode plate on a selected portion of first sacrificial layer, the electrode plate having an outer boundary; forming a second sacrificial layer on the electrode plate, the second sacrificial layer extending beyond the outer boundary of the electrode plate; forming a hole in the second sacrificial layer down to the electrode plate; filling the hole with a material that connects to the electrode plate, thereby forming a post structure; forming a support member on the second sacrificial layer including over the post structure, wherein the support member connects to the post structure; and removing the first and second sacrificial layers, thereby leaving a cavity around the electrode plate and at least a portion of the post structure.
It is contemplated that the second sacrificial layer may have an outer boundary, and that the support plate may extend beyond that outer boundary and down to the substrate. In this configuration, the support plate and the substrate may form a cavity.
In the illustrative method, the cavity may have one or more etching channels extending between the cavity and the exterior of the cavity. The etching channels may allow the removing step to remove the first and second sacrificial layers through the etching channels using a known process. After the first and second sacrificial layers are removed, the etching channels may be sealed. It is contemplated that the sealing step may be performed in a predetermined pressure environment to provide a predetermined pressure inside the cavity.